In a method of this type known from U.S. Pat. No. 5,016,681 the coils are supplied with a voltage which is essentially constant during the whole motion of the stopper element. The voltage is high in order to achieve short motion times, for example of the magnitude of 5 ms for a motion of the magnitude of 4 mm. After the motion and a bounce, if any, to/in any end position, the voltage is usually decreased to an essentially lower value so that a suitable holding force is achieved without any overheating (in the long run). It is also usual that the motion- and/or holding voltage is controlled in order to compensate for the temperature-dependence of the stopper magnet. This type of power supply causes several limitations when the motion times are very short. The inductance of the stopper magnet gives an electric time constant which can be of the same magnitude as the motion time. The current, and subsequently also the force in the stopper magnet, will then rise relatively slowly. The consequence of this will, on one hand, be a time loss before the motion starts and, on the other hand, also a slow acceleration with a further time loss in the beginning of the motion. Furthermore, the force of the stopper magnet is usually position-dependent. At a certain current the force increases, and thereby also the acceleration of the armature, essentially when the armature is approaching its end position. This will cause the final speed of the armature to be high, often in the magnitude of 4 m/s. A short motion time means a high supplied energy amount with a high temperature as a consequence. A short motion time means also a high final speed with a high load at the end position as a consequence. The end position dampers are furthermore usually of a material, the load capability of which decreases drastically at an increasing temperature.
In solutions according to the prior art, usually a mechanical spring was used to keep the stopper magnet in any of the end positions in a current-free state. On stopper magnets with only one coil, this spring was usually used also for the return motion of the stopper element. This design has disadvantages, because the spring will cause a risk for mechanical wear and a not-inessential decrease of the force which is available for the motion.
According to a method as known from U.S. Pat. No. 4,310,868, a solenoid equipped with a driver circuit is actuated for each of consecutive picking strokes by first supplying very high current, which current is maintained relatively high over the entire picking stroke of the armature. Since the time constant of the pick capacitor circuit, i.e. the value of a resistor times the value of a capacitor, is much greater than the time constant of the solenoid, the drive circuit will hold strong current much longer than needed to build up a strong current in the solenoid. The current is decisive for the transmitted force. There is relatively strong current, i.e. high force, even when the armature has reached the end position. As a further consequence of the time constant of the pick capacitor circuit increased voltage is supplied to the coil over the entire picking stroke of the armature.
It is an object of the present invention to achieve a short motion cycle of the stopper magnet with a low input energy amount and a relatively low final speed (kinetic energy), and to reduce the demand for control in order to compensate for the temperature dependence of the stopper magnet. Additionally, the risk for mechanical wear ought to be reduced while a sufficiently strong holding force ought to be maintained in the end position of the stopper magnet.
During the initial start part of the motion cycle of the stopper magnet, the electromagnetic coil is supplied with a voltage, which may be constant or may vary, which is considerably higher than the average voltage level during the remaining part of the motion cycle. Due to this increased voltage supplied in the initial start part of the motion cycle, the magnetic field in the coil builds up very quickly. Thus, the motion of the movable parts of the stopper magnet starts comparatively early. Moreover, due to the increased voltage in the initial start part of the motion cycle the accelerating force for the armature is very high at the beginning of the motion of the stopper magnet. This high acceleration further reduces time losses at the beginning of the motion cycle.
By providing a permanent magnet mounted to the yarn stopper element and soft iron magnetic material in a fixed part of the stopper magnet, a holding force is achieved for the yarn stopper element in the end position of the stopper magnet by the magnetic attraction between the permanent magnet and the soft iron magnetic material. The movable parts can be held in the end position of the stopper magnet without physical contact with the fixed parts, and, thus, without friction or wear. The stopper magnet can preferably be operated by the above-mentioned method for reducing the input energy amount and the final speed.
The units of time, current and position in FIGS. 2 and 3 are arbitrary.